A basic component of a printed circuit board is a dielectric layer having a sheet of copper foil bonded thereto. Through a subtractive process that includes one or more etching steps, portions of the copper foil are etched away to leave a distinct pattern of conductive lines and formed elements on the surface of the dielectric layer. Multi-layer printed circuit boards are formed by stacking and joining two or more of the aforementioned dielectric layers having printed circuits thereon.
In recent years, the trend has been to reduce the size of electronic components and provide printed circuit boards having multi-chip modules, etc. This results in a need to increase the number of components, i.e., surface mount components, provided on a printed circuit board. A key to providing a densely populated circuit board is to produce close and fine circuit patterns from the copper. The width and spacing of conductive paths on the printed circuit board are generally dictated by the thickness of the copper on the dielectric layer. For example, if the copper has a thickness of 35 .mu.m (which is a conventional 1-ounce copper used in manufacturing many printed circuits), exposing the printed circuit board to an etching process for a period of time to remove such a copper thickness will also reduce the width of the side areas of the printed circuit path in approximately the same amount. In other words, the width of the spacing between adjacent circuit lines is basically determined by the thickness of the copper foil on the dielectric layer, as well as the narrowness of the line to be formed. In this respect, as the thickness of the foil decreases below 35 .mu.m, the ability to physically handle such foil becomes more difficult. There is a minimum thickness of a copper foil sheet that is determined by the ability of the foil manufacturer to handle and transport such sheets. Hence, there is also a limit on achieving close and fine circuit patterns from copper sheets.
In recent years, it has been proposed to use copper-coated polyimides in forming printed circuits. The thickness of the copper on polyimide is generally significantly less than the thickness of traditional copper foil sheet. The thinner copper on the polyimide allows for finer and more closely spaced circuit lines in that the thinness of the copper layer reduces the etching time required to remove unwanted copper. In this respect, it is possible to use copper clad polyimide wherein the copper has a thickness as low as 0.1 .mu.m (1,000 .ANG.). The thinner copper on the polyimide also finds advantageous application in a semi-additive process. In a semi-additive process, the copper is masked to define a circuit pattern, and copper is plated onto the exposed pattern to build up a circuit. The mask material is removed and a "flash etch" removes the base copper on the polyimide leaving the built-up circuit on the polyimide. Thus, copper on polyimide finds advantageous application in both subtractive and semi-additive processes for forming printed circuits.
As with conventional copper foil, it is important to maintain the surface of the copper on the polyimide free from contamination. It is also important not to bend or flex the polyimide in such a way that cracks may form in the copper layer. In this respect, it is preferable to maintain a copper coated polyimide sheet in a flat planer orientation, i.e. the orientation that will be used within a printed circuit.
The present invention provides a method of protecting the exposed copper layer on a copper-coated polyimide from surface contamination and from undesirable flexing of the laminate.